hnRNP A2/B1 interacts with influenza A viral protein NS1 and inhibits virus replication potentially through suppressing NS1 RNA/protein levels and NS1 mRNA nuclear export

The interferon-regulated host factor hnRNPA0 modulates HIV-1 production by interference with LTR activity, mRNA trafficking, and programmed ribosomal frameshifting

The interplay between host factors and viral components impacts viral replication efficiency profoundly. Members of the cellular heterogeneous nuclear ribonucleoprotein family (hnRNPs) have been extensively studied as HIV-1 host dependency factors, but whether they play a role in innate immunity is currently unknown. This study aimed to identify hnRNPA0 as a type I interferon (IFN)-repressed host factor in HIV-1-infected cells. Knockdown of hnRNPA0, a situation that mirrors conditions under IFN stimulation, increased LTR activity, export of unspliced HIV-1 mRNA, viral particle production, and thus, increased infectivity. Conversely, hnRNPA0 overexpression primarily reduced plasmid-driven and integrated HIV-1 long terminal repeat (LTR) activity, significantly decreasing total viral mRNA and protein levels. In addition, high levels of hnRNPA0 significantly reduced the HIV-1 programmed ribosomal frameshifting efficiency, resulting in a shift in the HIV-1 p55/p15 ratio. The HIV-1 alternative splice site usage remained largely unaffected by altered hnRNPA0 levels suggesting that the synergistic inhibition of the LTR activity and viral mRNA transcription, as well as impaired ribosomal frameshifting efficiency, are critical factors for efficient HIV-1 replication regulated by hnRNPA0. The pleiotropic dose-dependent effects under high or low hnRNPA0 levels were further confirmed in HIV-1-infected Jurkat cells. Finally, our study revealed that hnRNPA0 levels in PBMCs were lower in therapy-naive HIV-1-infected individuals compared to healthy controls. Our findings highlight a significant role for hnRNPA0 in HIV-1 replication and suggest that its IFN-I-regulated expression levels are critical for viral fitness allowing replication in an antiviral environment.IMPORTANCERNA-binding proteins, in particular, heterogeneous nuclear ribonucleoproteins (hnRNPs), have been extensively studied. Some act as host dependency factors for HIV-1 since they are involved in multiple cellular gene expression processes. Our study revealed hnRNPA0 as an IFN-regulated host factor, that is differently expressed after IFN-I treatment in HIV-1 target cells and lower expressed in therapy-naïve HIV-1-infected individuals. Our findings demonstrate the significant pleiotropic role of hnRNPA0 in viral replication: In high concentrations, hnRNPA0 limits viral replication by negatively regulating Tat-LTR transcription, retaining unspliced mRNA in the nucleus, and significantly impairing programmed ribosomal frameshifting. Low hnRNPA0 levels as observed in IFN-treated THP-1 cells, particularly facilitate HIV LTR activity and unspliced mRNA export, suggesting a role in innate immunity in favor of HIV replication. Understanding the mode of action between hnRNPA0 and HIV-1 gene expression might help to identify novel therapeutically strategies against HIV-1 and other viruses.

The RBPome of influenza A virus NP-mRNA reveals a role for TDP-43 in viral replication

Genome-wide approaches have significantly advanced our knowledge of the repertoire of RNA-binding proteins (RBPs) that associate with cellular polyadenylated mRNAs within eukaryotic cells. Recent studies focusing on the RBP interactomes of viral mRNAs, notably SARS-Cov-2, have revealed both similarities and differences between the RBP profiles of viral and cellular mRNAs. However, the RBPome of influenza virus mRNAs remains unexplored. Herein, we identify RBPs that associate with the viral mRNA encoding the nucleoprotein (NP) of an influenza A virus. Focusing on TDP-43, we show that it binds several influenza mRNAs beyond the NP-mRNA, and that its depletion results in lower levels of viral mRNAs and proteins within infected cells, and a decreased yield of infectious viral particles. We provide evidence that the viral polymerase recruits TDP-43 onto viral mRNAs through a direct interaction with the disordered C-terminal domain of TDP-43. Notably, other RBPs found to be associated with influenza virus mRNAs also interact with the viral polymerase, which points to a role of the polymerase in orchestrating the assembly of viral messenger ribonucleoproteins.

Targeting HNRNPA2B1 inhibits enterovirus 71 replication in SK-N-SH cells

OBJECTIVE

To investigate the effect of heterogeneous nuclear ribonucleoprotein A2B1 (HNRNPA2B1) on the replication of enterovirus 71 (EV-71) in SK-N-SH cells.

METHODS

The mRNA and protein expression of HNRNPA2B1 in SK-N-SH cells were detected by real-time quantitative PCR (qRT-PCR) and western blotting (WB), respectively. WB was used to detect HNRNPA2B1 protein expression in the nucleus and cytosol. The localization of HNRNPA2B1 protein in the nucleus and cytosol was detected by immunofluorescence (IF). The expression of HNRNPA2B1 was inhibited by small interfering RNA (si-HNRNPA2B1). Viral RNA, viral structural protein VP1, and viral titer were detected by qRT-PCR, WB, and viral dilution counting, respectively.

RESULTS

EV-71 infection significantly upregulates the expression of HNRNPA2B1 in SK-N-SH cells. EV-71 infection promotes HNRNPA2B1 nucleus-cytoplasm redistribution. Down-regulation of HNRNPA2B1 expression significantly inhibited EV-71 replication.

CONCLUSION

HNRNPA2B1 protein redistributed from nucleus to cytoplasm and is highly expressed in the cytoplasm during EV-71 infection. Inhibition of HNRNPA2B1 levels effectively inhibits EV-71 replication in SK-N-SH cells.

Upregulation of galectin-3 in influenza A virus infection promotes viral RNA synthesis through its association with viral PA protein

BACKGROUND

Influenza is one of the most important viral infections globally. Viral RNA-dependent RNA polymerase (RdRp) consists of the PA, PB1, and PB2 subunits, and the amino acid residues of each subunit are highly conserved among influenza A virus (IAV) strains. Due to the high mutation rate and emergence of drug resistance, new antiviral strategies are needed. Host cell factors are involved in the transcription and replication of influenza virus. Here, we investigated the role of galectin-3, a member of the β-galactoside-binding animal lectin family, in the life cycle of IAV infection in vitro and in mice.

METHODS

We used galectin-3 knockout and wild-type mice and cells to study the intracellular role of galectin-3 in influenza pathogenesis. Body weight and survival time of IAV-infected mice were analyzed, and viral production in mouse macrophages and lung fibroblasts was examined. Overexpression and knockdown of galectin-3 in A549 human lung epithelial cells were exploited to assess viral entry, viral ribonucleoprotein (vRNP) import/export, transcription, replication, virion production, as well as interactions between galectin-3 and viral proteins by immunoblotting, immunofluorescence, co-immunoprecipitation, RT-qPCR, minireplicon, and plaque assays. We also employed recombinant galectin-3 proteins to identify specific step(s) of the viral life cycle that was affected by exogenously added galectin-3 in A549 cells.

RESULTS

Galectin-3 levels were increased in the bronchoalveolar lavage fluid and lungs of IAV-infected mice. There was a positive correlation between galectin-3 levels and viral loads. Notably, galectin-3 knockout mice were resistant to IAV infection. Knockdown of galectin-3 significantly reduced the production of viral proteins and virions in A549 cells. While intracellular galectin-3 did not affect viral entry, it increased vRNP nuclear import, RdRp activity, and viral transcription and replication, which were associated with the interaction of galectin-3 with viral PA subunit. Galectin-3 enhanced the interaction between viral PA and PB1 proteins. Moreover, exogenously added recombinant galectin-3 proteins also enhanced viral adsorption and promoted IAV infection in A549 cells.

CONCLUSION

We demonstrate that galectin-3 enhances viral infection through increases in vRNP nuclear import and RdRp activity, thereby facilitating viral transcription and replication. Our findings also identify galectin-3 as a potential therapeutic target for influenza.

Structural Insight Into hnRNP A2/B1 Homodimerization and DNA Recognition

Heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNP A2/B1) has been identified as a nuclear DNA sensor. Upon viral infection, hnRNP A2/B1 recognizes pathogen-derived DNA as a homodimer, which is a prerequisite for its translocation to the cytoplasm to activate the interferon response. However, the DNA binding mechanism inducing hnRNP A2/B1 homodimerization is unknown. Here, we show the crystal structure of the RNA recognition motif (RRM) of hnRNP A2/B1 in complex with a U-shaped ssDNA, which mediates the formation of a newly observed protein dimer. Our biochemical assays and mutagenesis studies confirm that the hnRNP A2/B1 homodimer forms in solution by binding to pre-generated ssDNA or dsDNA with a U-shaped bulge. These results depict a potential functional state of hnRNP A2/B1 in antiviral immunity and other cellular processes.

Diverse roles of heterogeneous nuclear ribonucleoproteins in viral life cycle

Understanding the host-virus interactions helps to decipher the viral replication strategies and pathogenesis. Viruses have limited genetic content and rely significantly on their host cell to establish a successful infection. Viruses depend on the host for a broad spectrum of cellular RNA-binding proteins (RBPs) throughout their life cycle. One of the major RBP families is the heterogeneous nuclear ribonucleoproteins (hnRNPs) family. hnRNPs are typically localized in the nucleus, where they are forming complexes with pre-mRNAs and contribute to many aspects of nucleic acid metabolism. hnRNPs contain RNA binding motifs and frequently function as RNA chaperones involved in pre-mRNA processing, RNA splicing, and export. Many hnRNPs shuttle between the nucleus and the cytoplasm and influence cytoplasmic processes such as mRNA stability, localization, and translation. The interactions between the hnRNPs and viral components are well-known. They are critical for processing viral nucleic acids and proteins and, therefore, impact the success of the viral infection. This review discusses the molecular mechanisms by which hnRNPs interact with and regulate each stage of the viral life cycle, such as replication, splicing, translation, and assembly of virus progeny. In addition, we expand on the role of hnRNPs in the antiviral response and as potential targets for antiviral drug research and development.

The Network of Interactions between the Porcine Epidemic Diarrhea Virus Nucleocapsid and Host Cellular Proteins

Host–virus protein interactions are critical for intracellular viral propagation. Understanding the interactions between cellular and viral proteins may help us develop new antiviral strategies. Porcine epidemic diarrhea virus (PEDV) is a highly contagious coronavirus that causes severe damage to the global swine industry. Here, we employed co-immunoprecipitation and liquid chromatography-mass spectrometry to characterize 426 unique PEDV nucleocapsid (N) protein-binding proteins in infected Vero cells. A protein–protein interaction network (PPI) was created, and gene ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) database analyses revealed that the PEDV N-bound proteins belong to different cellular pathways, such as nucleic acid binding, ribonucleoprotein complex binding, RNA methyltransferase, and polymerase activities. Interactions of the PEDV N protein with 11 putative proteins: tripartite motif containing 21, DEAD-box RNA helicase 24, G3BP stress granule assembly factor 1, heat shock protein family A member 8, heat shock protein 90 alpha family class B member 1, YTH domain containing 1, nucleolin, Y-box binding protein 1, vimentin, heterogeneous nuclear ribonucleoprotein A2/B1, and karyopherin subunit alpha 1, were further confirmed by in vitro co-immunoprecipitation assay. In summary, studying an interaction network can facilitate the identification of antiviral therapeutic strategies and novel targets for PEDV infection.

Viral-host interactions during splicing and nuclear export of influenza virus mRNAs

As influenza-A viruses (IAV) replicate in the host cell nucleus, intranuclear pathways are usurped for viral gene expression. The eight genomic viral ribonucleoproteins (vRNPs) segments of IAV are transcribed and two generate viral mRNAs (M and NS) that undergo alternative splicing followed by export from the nucleus. The focus of this review is on viral RNA splicing and nuclear export. Recent mechanistic advances on M and NS splicing show differential regulation by RNA-binding proteins as well as distinct intranuclear localization. After a review of IAV splicing, we will discuss the nuclear export of viral mRNAs, which occur by interacting with specific constituents of the host mRNA export machinery that translocate viral mRNAs through the nuclear pore complex for translation in the cytoplasm.

Cap-snatching as a possible contributor to photosynthesis shut-off

Cap-snatching is a mechanism applied by segmented, negative strand (-) RNA viruses (NSVs) to initiate genome transcription. So far, the cap donor source of cytoplasmic-replicating NSVs has remained elusive. Recently, studies pointed to processing body (P body, PB) as the potential source for providing capped RNAs but conclusive evidence is still lacking. To attempt identifying these sources, here the 5' non-viral leader sequences of Tomato spotted wilt virus (TSWV) N mRNAs were analysed by high-throughput sequencing (HTS) from plants subjected to normal and heat-stress conditions, and subsequently mapped on host donor transcripts. The majority of non-viral heterogenous, host-derived leader sequences ranged in size between ~10-20 nt and contained A or AG residues at the cleavage site and the presence of certain sequence motifs. Mapping the capped-leader sequences to the 5' UTR region of genes encoded by the Nicotiana tabacum genome, identified 348 donor genes and which were specifically enriched in cellular photosynthesis pathway. Nineteen of those were clearly expressed differentially at normal condition versus heat-stress conditions. Although the results did not point towards snatching of capped-RNA leader sequences from certain cytoplasmic RNA granules in particular, they indicated photosynthesis downregulation (and development of disease symptoms) partially result from cap-snatching.

Interaction Network of Porcine Circovirus Type 3 and 4 Capsids with Host Proteins

An extensive understanding of the interactions between host cellular and viral proteins provides clues for studying novel antiviral strategies. Porcine circovirus type 3 (PCV3) and type 4 (PCV4) have recently been identified as viruses that can potentially damage the swine industry. Herein, 401 putative PCV3 Cap-binding and 484 putative PCV4 Cap-binding proteins were characterized using co-immunoprecipitation and liquid chromatography-mass spectrometry. Both PCV3 and PCV4 Caps shared 278 identical interacting proteins, but some putative interacting proteins (123 for PCV3 Cap and 206 for PCV4 Cap) differed. A protein-protein interaction network was constructed, and according to gene ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) database analyses, both PCV3 Cap- and PCV4 Cap-binding proteins participated mainly in ribosome biogenesis, nucleic acid binding, and ATP-dependent RNA helicase activities. Verification assays of eight putative interacting proteins indicated that nucleophosmin-1, nucleolin, DEAD-box RNA helicase 21, heterogeneous nuclear ribonucleoprotein A2/B1, YTH N6-methyladenosine RNA binding protein 1, and Y-box binding protein 1 bound directly to both PCV3 and PCV4 Caps, but ring finger protein 2 and signal transducer and activator of transcription 6 did not. Therefore, the interaction network provided helpful information to support further research into the underlying mechanisms of PCV3 and PCV4 infection.

Cellular hnRNPAB interacts with avian influenza viral protein PB2 and inhibits virus replication potentially by restricting PB2 mRNA nuclear export and PB2 protein level

The PB2 protein of avian influenza virus (AIV) is essential for transcription and replication of virus genome. In this study, we reported that chicken heterogenous nuclear riboncleoprotein AB (hnRNPAB) cooperated with avian influenza viral protein PB2 and inhibited the polymerase activity and virus replication. We found that hnRNPAB was associated with PB2 mRNA and overexpression of hnRNPAB reduced PB2 mRNA nuclear export and PB2 protein level, but had no influence on PB2 mRNA level. At the same time, overexpression of hnRNPAB also reduced protein levels rather than mRNA levels of PA, PB1 and NP. In addition, overexpression of hnRNPAB restricted the polymerase activity and virus replication, while knockdown of hnRNPAB resulted in enhanced polymerase activity and virus replication. Lastly, virus infection induced the nuclear accumulation of hnRNPAB, but did not cause the change of expression level of endogenous hnRNPAB in DF-1 cells. Collectively, these findings suggested that hnRNPAB played a restrictive role in polymerase activity and virus replication potentially through inhibiting PB2 mRNA nuclear export and PB2 protein level.

RuvB-Like Protein 2 Interacts with the NS1 Protein of Influenza A Virus and Affects Apoptosis That Is Counterbalanced by Type I Interferons

The NS1 protein of influenza A virus (IAV) plays important roles in viral pathogenesis and host immune response. Through a proteomic approach, we have identified RuvB-like proteins 1 and 2 (RuvBL1 and RuvBL2) as interacting partners of the NS1 protein of IAVs. Infection of human lung A549 cells with A/PR/8/34 (PR8) virus resulted in reductions in the protein levels of RuvBL2 but not RuvBL1. Further studies with RuvBL2 demonstrated that the NS1-RuvBL2 interaction is RNA-independent, and RuvBL2 binds the RNA-binding domain of the NS1. Infection of interferon (IFN)-deficient Vero cells with wild-type or delNS1 PR8 virus reduced RuvBL2 protein levels and induced apoptosis; delNS1 virus caused more reductions in RuvBL2 protein levels and induced more apoptosis than did wild-type virus. Knockdown of RuvBL2 by siRNAs induced apoptosis and overexpression of RuvBL2 resulted in increased resistance to infection-induced apoptosis in Vero cells. These results suggest that a non-NS1 viral element or elements induce apoptosis by suppressing RuvBL2 protein levels, and the NS1 inhibits the non-NS1 viral element-induced apoptosis by maintaining RuvBL2 abundance in infected cells in the absence of IFN influence. In contrast to Vero cells, infection of IFN-competent A549 cells with PR8 virus caused reductions in RuvBL2 protein levels but did not induce apoptosis. Concomitantly, pretreatment of Vero cells with a recombinant IFN resulted in resistance to infection-induced apoptosis. These results demonstrate that the infection-induced, RuvBL2-regulated apoptosis in infected cells is counterbalanced by IFN survival signals. Our results reveal a novel mechanism underlying the infection-induced apoptosis that can be modulated by the NS1 and type I IFN signaling in IAV-infected cells.

Cellular hnRNPAB binding to viral nucleoprotein inhibits flu virus replication by blocking nuclear export of viral mRNA

Heterogeneous nuclear ribonucleoproteins (hnRNPs) play critical roles in the nuclear export, splicing, and sensing of RNA. However, the role of heterogeneous nuclear ribonucleoprotein A/B (hnRNPAB) is poorly understood. In this study, we report that hnRNPAB cooperates with nucleoprotein (NP) to restrict viral mRNA nuclear export via inhibiting viral mRNA binding to ALY and NXF1. HnRNPAB restricts mRNA transfer from ALY to NXF1, inhibiting the mRNA nuclear export. Moreover, when cells are invaded by influenza A virus, NP interacts with hnRNPAB and interrupts the ALY-UAP56 interaction, leading to repression of ALY-viral mRNA binding, and then inhibits the viral mRNA binding to NXF1, leading to nuclear stimulation of viral mRNA. Collectively, these observations provide a new role of hnRNPAB to act as an mRNA nuclear retention factor, which is also effective for viral mRNA of influenza A virus, and NP cooperates with hnRNPAB to further restrict the viral mRNA nuclear export.

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HnRNPAB inhibits influenza A virus replication by repressing viral mRNA nuclear export

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HnRNPAB interrupts viral mRNA transferring from ALY to NXF1

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NP cooperates with hnRNPAB to further restrict viral mRNA nuclear export

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The ALY-viral mRNA binding is restricted by NP-hnRNPAB complex

Investigating Ketone Bodies as Immunometabolic Countermeasures against Respiratory Viral Infections

Respiratory viral infections remain a scourge, with seasonal influenza infecting millions and killing many thousands annually and viral pandemics, such as COVID-19, recurring every decade. Age, cardiovascular disease, and diabetes mellitus are risk factors for severe disease and death from viral infection. Immunometabolic therapies for these populations hold promise to reduce the risks of death and disability. Such interventions have pleiotropic effects that might not only target the virus itself but also enhance supportive care to reduce cardiopulmonary complications, improve cognitive resilience, and facilitate functional recovery. Ketone bodies are endogenous metabolites that maintain cellular energy but also feature drug-like signaling activities that affect immune activity, metabolism, and epigenetics. Here, we provide an overview of ketone body biology relevant to respiratory viral infection, focusing on influenza A and severe acute respiratory syndrome (SARS)-CoV-2, and discuss the opportunities, risks, and research gaps in the study of exogenous ketone bodies as novel immunometabolic interventions in these diseases.

Anti-Influenza Activity of the Ribonuclease Binase: Cellular Targets Detected by Quantitative Proteomics

Unpredictable influenza pandemics, annual epidemics, and sporadic poultry-to-human avian influenza virus infections with high morbidity and mortality rates dictate a need to develop new antiviral approaches. Targeting cellular pathways and processes is a promising antiviral strategy shown to be effective regardless of viral subtypes or viral evolution of drug-resistant variants. Proteomics-based searches provide a tool to reveal the druggable stages of the virus life cycle and to understand the putative antiviral mode of action of the drug(s). Ribonucleases (RNases) of different origins not only demonstrate antiviral effects that are mediated by the direct RNase action on viral and cellular RNAs but can also exert their impact by signal transduction modulation. To our knowledge, studies of the RNase-affected cell proteome have not yet been performed. To reveal cellular targets and explain the mechanisms underlying the antiviral effect employed by the small extra-cellular ribonuclease of Bacillus pumilus (binase) both in vitro and in vivo, qualitative shotgun and quantitative targeted proteomic analyses of the influenza A virus (IAV) H1N1pdm09-infected A549 cells upon binase treatment were performed. We compared proteomes of mock-treated, binase-treated, virus-infected, and virus-infected binase-treated cells to determine the proteins affected by IAV and/or binase. In general, IAV demonstrated a downregulating strategy towards cellular proteins, while binase had an upregulating effect. With the help of bioinformatics approaches, coregulated cellular protein sets were defined and assigned to their biological function; a possible interconnection with the progression of viral infection was conferred. Most of the proteins downregulated by IAV (e.g., AKR1B1, AKR1C1, CCL5, PFN1, RAN, S100A4, etc.) belong to the processes of cellular metabolism, response to stimulus, biological regulation, and cellular localization. Upregulated proteins upon the binase treatment (e.g., AKR1B10, CAP1, HNRNPA2B1, PFN1, PPIA, YWHAB, etc.) are united by the processes of biological regulation, cellular localization, and immune and metabolic processes. The antiviral activity of binase against IAV was expressed by the inversion of virus-induced proteomic changes, resulting in the inhibition of virus-associated processes, including nuclear ribonucleoprotein export (NCL, NPM1, Nup205, and Bax proteins involved) and cytoskeleton remodeling (RDX, PFN1, and TUBB) induced by IAV at the middle stage of single-cycle infection in A549 cells. Modulation of the immune response could be involved as well. Overall, it seems possible that binase exerts its antiviral effects in multiple ways.

Stress proteins: the biological functions in virus infection, present and challenges for target-based antiviral drug development

Stress proteins (SPs) including heat-shock proteins (HSPs), RNA chaperones, and ER associated stress proteins are molecular chaperones essential for cellular homeostasis. The major functions of HSPs include chaperoning misfolded or unfolded polypeptides, protecting cells from toxic stress, and presenting immune and inflammatory cytokines. Regarded as a double-edged sword, HSPs also cooperate with numerous viruses and cancer cells to promote their survival. RNA chaperones are a group of heterogeneous nuclear ribonucleoproteins (hnRNPs), which are essential factors for manipulating both the functions and metabolisms of pre-mRNAs/hnRNAs transcribed by RNA polymerase II. hnRNPs involve in a large number of cellular processes, including chromatin remodelling, transcription regulation, RNP assembly and stabilization, RNA export, virus replication, histone-like nucleoid structuring, and even intracellular immunity. Dysregulation of stress proteins is associated with many human diseases including human cancer, cardiovascular diseases, neurodegenerative diseases (e.g., Parkinson’s diseases, Alzheimer disease), stroke and infectious diseases. In this review, we summarized the biologic function of stress proteins, and current progress on their mechanisms related to virus reproduction and diseases caused by virus infections. As SPs also attract a great interest as potential antiviral targets (e.g., COVID-19), we also discuss the present progress and challenges in this area of HSP-based drug development, as well as with compounds already under clinical evaluation.

Heterogeneous Nuclear Ribonucleoproteins Participate in Hepatitis E Virus Replication

Coordinated assembly of viral and host factors is essential for the successful propagation of viruses as well as the generation of host antiviral response. Previous studies from our group, as well as from other groups, have identified host proteins interacting with various components of the hepatitis E virus (HEV). However, the functional relevance of host protein interactions in HEV replication context has been notably overlooked. The present study reports that heterogeneous nuclear ribonucleoproteins (hnRNPs), namely hnRNPK, hnRNPA2B1, hnRNPH, PCBP1 and PCBP2, interact with HEV RNA promoter and RNA-dependent RNA polymerase to regulate HEV replication. We found that hnRNPK and hnRNPA2B1 are the virus-supportive factors interacting with HEV RNA at promoter regions along with HEV polymerase protein, which are essential for HEV replication in the cells. Contrarily, hnRNPH, PCBP1 and PCBP2 are the antiviral factors that interact exclusively with HEV genomic promoter and inhibit HEV replication in Huh7 S10-3 cells. In vitro RNA-binding assays revealed that the antiviral hnRNP proteins hamper the binding of virus-supportive hnRNP proteins at HEV genomic promoter. In the binding reaction, the binding of HEV polymerase protein to the genomic promoter is slightly affected by the presence of antiviral hnRNPH. In an effort of visualizing the subcellular localization of hnRNP proteins in the HEV replication scenario in the Huh7 cells, we showed that hnRNPK, hnRNPA2B1, hnRNPH, PCBP1 and PCBP2 redistribute from nucleus to cytoplasm. In conclusion, our study highlights the importance of hnRNP proteins in HEV replication regulation.

Upregulation of CD73 Confers Acquired Radioresistance and is Required for Maintaining Irradiation-selected Pancreatic Cancer Cells in a Mesenchymal State

The molecular mechanisms underlying exceptional radioresistance in pancreatic cancer remain elusive. In the present study, we established a stable radioresistant pancreatic cancer cell line MIA PaCa-2-R by exposing the parental MIA PaCa-2 cells to fractionated ionizing radiation (IR). Systematic proteomics and bioinformatics analysis of protein expression in MIA PaCa-2 and MIA PaCa-2-R cells revealed that several growth factor-/cytokine-mediated pathways, including the OSM/STAT3, PI3K/AKT, and MAPK/ERK pathways, were activated in the radioresistant cells, leading to inhibition of apoptosis and increased epithelial-mesenchymal plasticity. In addition, the radioresistant cells exhibited enhanced capabilities of DNA repair and antioxidant defense compared with the parental cells. We focused functional analysis on one of the most up-regulated proteins in the radioresistant cells, ecto-5'-nucleotidase (CD73), which is a cell surface protein that is overexpressed in different types of cancer. Ectopic overexpression of CD73 in the parental cells resulted in radioresistance and conferred resistance to IR-induced apoptosis. Knockdown of CD73 re-sensitized the radioresistant cells to IR and IR-induced apoptosis. The effect of CD73 on radioresistance and apoptosis is independent of the enzymatic activity of CD73. Further studies demonstrate that CD73 up-regulation promotes Ser-136 phosphorylation of the proapoptotic protein BAD and is required for maintaining the radioresistant cells in a mesenchymal state. Our findings suggest that expression alterations in the IR-selected pancreatic cancer cells result in hyperactivation of the growth factor/cytokine signaling that promotes epithelial-mesenchymal plasticity and enhancement of DNA repair. Our results also suggest that CD73, potentially a novel downstream factor of the enhanced growth factor/cytokine signaling, confers acquired radioresistance by inactivating proapoptotic protein BAD via phosphorylation of BAD at Ser-136 and by maintaining the radioresistant pancreatic cancer cells in a mesenchymal state.

Protein Interactions Network of Hepatitis E Virus RNA and Polymerase With Host Proteins

Host-pathogen interactions are crucial for the successful propagation of pathogens inside the host cell. Knowledge of interactions between host proteins and viral proteins or viral RNA may provide clues for developing novel antiviral strategies. Hepatitis E virus (HEV), a water-borne pathogen that causes acute hepatitis in humans, is responsible for epidemics in developing countries. HEV pathology and molecular biology have been poorly explored due to the lack of efficient culture systems. A contemporary approach, to better understand the viral infection cycle at the molecular level, is the use of system biology tools depicting virus-host interactions. To determine the host proteins which participate in the regulation of HEV replication, we indentified liver cell proteins interacting with HEV RNA at its putative promoter region and those interacting with HEV polymerase (RdRp) protein. We employed affinity chromatography followed by liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS) to identify the interacting host proteins. Protein-protein interaction networks (PPI) were plotted and analyzed using web-based tools. Topological analysis of the network revealed that the constructed network is potentially significant and relevant for viral replication. Gene ontology and pathway enrichment analysis revealed that HEV RNA promoter- and polymerase-interacting host proteins belong to different cellular pathways such as RNA splicing, RNA metabolism, protein processing in endoplasmic reticulum, unfolded protein response, innate immune pathways, secretory vesicle pathway, and glucose metabolism. We showed that hnRNPK and hnRNPA2B1 interact with both HEV putative promoters and HEV RdRp, which suggest that they may have crucial roles in HEV replication. We demonstrated in vitro binding of hnRNPK and hnRNPA2B1 proteins with the HEV targets in the study, assuring the authenticity of the interactions obtained through mass spectrometry. Thus, our study highlights the ability of viruses, such as HEV, to maneuver host systems to create favorable cellular environments for virus propagation. Studying the host-virus interactions can facilitate the identification of antiviral therapeutic strategies and novel targets.

Host Determinants of Influenza RNA Synthesis

Influenza viruses are a leading cause of seasonal and pandemic respiratory illness. Influenza is a negative-sense single-stranded RNA virus that encodes its own RNA-dependent RNA polymerase (RdRp) for nucleic acid synthesis. The RdRp catalyzes mRNA synthesis, as well as replication of the virus genome (viral RNA) through a complementary RNA intermediate. Virus propagation requires the generation of these RNA species in a controlled manner while competing heavily with the host cell for resources. Influenza virus appropriates host factors to enhance and regulate RdRp activity at every step of RNA synthesis. This review describes such host factors and summarizes our current understanding of the roles they play in viral synthesis of RNA.

Functional roles of hnRNPA2/B1 regulated by METTL3 in mammalian embryonic development

Heterogeneous nuclear ribonucleoprotein A2/B1 (hnRNPA2/B1) plays an important role in RNA processing via in m⁶A modification of pre-mRNA or pre-miRNA. However, the functional role of and relationship between m⁶A and hnRNPA2/B1 in early embryonic development are unclear. Here, we found that hnRNPA2/B1 is crucial for early embryonic development by virtue of regulating specific gene transcripts. HnRNPA2/B1 was localized to the nucleus and cytoplasm during subsequent embryonic development, starting at fertilization. Knockdown of hnRNPA2/B1 delayed embryonic development after the 4-cell stage and blocked further development. RNA-Seq analysis revealed changes in the global expression patterns of genes involved in transcription, translation, cell cycle, embryonic stem cell differentiation, and RNA methylation in hnRNPA2/B1 KD blastocysts. The levels of the inner cell mass markers OCT4 and SOX2 were decreased in hnRNPA2/B1 KD blastocysts, whereas that of the differentiation marker GATA4 was decreased. N6-Adenosine methyltransferase METTL3 knock-down caused embryonic developmental defects similar to those in hnRNPA2/B1 KD embryos. Moreover, METTL3 KD blastocysts showed increased mis-localization of hnRNPA2/B1 and decreased m⁶A RNA methylation. Taken together, our results suggest that hnRNPA2/B1 is essential for early embryogenesis through the regulation of transcription-related factors and determination of cell fate transition. Moreover, hnRNPA2/B1 is regulated by METTL3-dependent m⁶A RNA methylation.

RNA regulatory processes in RNA virus biology

Numerous post‐transcriptional RNA processes play a major role in regulating the quantity, quality and diversity of gene expression in the cell. These include RNA processing events such as capping, splicing, polyadenylation and modification, but also aspects such as RNA localization, decay, translation, and non‐coding RNA‐associated regulation. The interface between the transcripts of RNA viruses and the various RNA regulatory processes in the cell, therefore, has high potential to significantly impact virus gene expression, regulation, cytopathology and pathogenesis. Furthermore, understanding RNA biology from the perspective of an RNA virus can shed considerable light on the broad impact of these post‐transcriptional processes in cell biology. Thus the goal of this article is to provide an overview of the richness of cellular RNA biology and how RNA viruses use, usurp and/or avoid the associated machinery to impact the outcome of infection.

This article is categorized under:

RNA in Disease and Development > RNA in Disease

Global Interactomics Connect Nuclear Mitotic Apparatus Protein NUMA1 to Influenza Virus Maturation

Influenza A virus (IAV) infections remain a major human health threat. IAV has enormous genetic plasticity and can rapidly escape virus-targeted anti-viral strategies. Thus, there is increasing interest to identify host proteins and processes the virus requires for replication and maturation. The IAV non-structural protein 1 (NS1) is a critical multifunctional protein that is expressed to high levels in infected cells. Host proteins that interact with NS1 may serve as ideal targets for attenuating IAV replication. We previously developed and characterized broadly cross-reactive anti-NS1 monoclonal antibodies. For the current study, we used these mAbs to co-immunoprecipitate native IAV NS1 and interacting host proteins; 183 proteins were consistently identified in this NS1 interactome study, 124 of which have not been previously reported. RNAi screens identified 11 NS1-interacting host factors as vital for IAV replication. Knocking down one of these, nuclear mitotic apparatus protein 1 (NUMA1), dramatically reduced IAV replication. IAV genomic transcription and translation were not inhibited but transport of viral structural proteins to the cell membrane was hindered during maturation steps in NUMA1 knockdown (KD) cells.

MicroRNA-451a in extracellular, blood-resident vesicles attenuates macrophage and dendritic cell responses to influenza whole-virus vaccine

The innate immune system is important for the efficacy of vaccines, but excessive innate immune responses can cause adverse reactions after vaccination. Extracellular vesicles (EVs) are enriched in the blood and can deliver functional RNAs, such as microRNAs (miRNAs), to recipient cells, thereby mediating intercellular communication. However, the role of EVs in controlling the innate immune responses to vaccines has not been fully elucidated. Here, we found that miR-451a is abundant in human serum EVs and that its presence in blood-circulating EVs affects the innate immune responses of macrophages and dendritic cells to inactivated whole-virus vaccines (WV) against influenza. miR-451a in human serum EVs was stable for a week in healthy subjects, and its levels gradually fluctuated over several months. miR-451a within serum EVs was internalized into serum-cultured macrophages and dendritic cells and reduced endogenous 14-3-3ζ protein levels and decreased the expression of type I IFN and interleukin 6 in response to WV stimulation. miR-451a levels in blood-circulating EVs were positively correlated with intracellular miR-451a levels in mouse splenic CD11c⁺ cells and inversely correlated with the innate immune response to inactivated WV in vivo These findings suggest that miR-451a in circulating EVs is internalized into recipient cells in vivo and that this internalization results in an attenuation of the innate immune response to WV. Moreover, a microarray analysis identified several other miRNAs that affect the macrophage response to inactivated WV. Our results reveal that miRNAs in circulating EVs significantly modify the responses of macrophages and dendritic cells to inactivated WV.

Cellular hnRNP A2/B1 interacts with the NP of influenza A virus and impacts viral replication

The viral ribonucleoprotein (vRNP) of influenza A virus is formed by virion RNA (vRNA), viral polymerase complex, and nucleoprotein (NP). The NP plays an important role in facilitating the replication and stabilization of viral RNA. To explore host factors that may be involved in the regulation of viral replication through interactions with NP, we conducted an immunoprecipitation experiment followed by mass spectrometry to identify NP-associated cellular proteins. Here, we demonstrate that NP can interact and colocalize with heterogeneous nuclear ribonucleoprotein (hnRNP) A2/B1 in mammalian cells and that the interaction may occur via direct binding to the glycine-rich domain (GRD) of hnRNP A2/B1. In addition, two residues in the tail loop of NP, F412 and R422, are required for the interaction of hnRNP A2/B1. Because the knockdown of hnRNP A2/B1 expression reduces viral RNP activity, hnRNP A2/B1 may act as a positive regulator in viral RNA synthesis of influenza A virus. More importantly, the findings in this research demonstrate that host proteins can regulate the replication of influenza A virus by interacting with NP.

iTRAQ-based Proteomic Analysis of Porcine Kidney Epithelial PK15 cells Infected with Pseudorabies virus

Pseudorabies virus (PRV) is one of the most important pathogens of swine, resulting in severe economic losses to the pig industry. To improve our understanding of the host responses to PRV infection, we applied isobaric tags for relative and absolute quantification (iTRAQ) labeling coupled with liquid chromatography-tandem mass spectrometry to quantitatively identify the differentially expressed cellular proteins in PRV-infected PK15 cells. In total, relative quantitative data were identified for 4333 proteins in PRV and mock- infected PK15 cells, among which 466 cellular proteins were differentially expressed, including 234 upregulated proteins and 232 downregulated proteins. Bioinformatics analysis disclosed that most of these differentially expressed proteins were involved in metabolic processes, cellular growth and proliferation, endoplasmic reticulum (ER) stress response, cell adhesion and cytoskeleton. Moreover, expression levels of four representative proteins, beta-catenin, STAT1, GRB2 and PCNA, were further confirmed by western blot analysis. This is the first attempt to analyze the protein profile of PRV-infected PK15 cells using iTRAQ technology, and our findings may provide valuable information to help understand the host response to PRV infection.

Networks of Host Factors that Interact with NS1 Protein of Influenza A Virus

Pigs are an important host of influenza A viruses due to their ability to generate reassortant viruses with pandemic potential. NS1 protein of influenza A viruses is a key virulence factor and a major antagonist of innate immune responses. It is also involved in enhancing viral mRNA translation and regulation of virus replication. Being a protein with pleiotropic functions, NS1 has a variety of cellular interaction partners. Hence, studies on swine influenza viruses (SIV) and identification of swine influenza NS1-interacting host proteins is of great interest. Here, we constructed a recombinant SIV carrying a Strep-tag in the NS1 protein and infected primary swine respiratory epithelial cells (SRECs) with this virus. The Strep-tag sequence in the NS1 protein enabled us to purify intact, the NS1 protein and its interacting protein complex specifically. We identified cellular proteins present in the purified complex by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and generated a dataset of these proteins. 445 proteins were identified by LC-MS/MS and among them 192 proteins were selected by setting up a threshold based on MS parameters. The selected proteins were analyzed by bioinformatics and were categorized as belonging to different functional groups including translation, RNA processing, cytoskeleton, innate immunity, and apoptosis. Protein interaction networks were derived using these data and the NS1 interactions with some of the specific host factors were verified by immunoprecipitation. The novel proteins and the networks revealed in our study will be the potential candidates for targeted study of the molecular interaction of NS1 with host proteins, which will provide insights into the identification of new therapeutic targets to control influenza infection and disease pathogenesis.

Quantification of the host response proteome after herpes simplex virus type 1 infection

Viruses employ numerous host cell metabolic functions to propagate and manage to evade the host immune system. For herpes simplex virus type 1 (HSV1), a virus that has evolved to efficiently infect humans without seriously harming the host in most cases, the virus-host interaction is specifically interesting. This interaction can be best characterized by studying the proteomic changes that occur in the host during infection. Previous studies have been successful at identifying numerous host proteins that play important roles in HSV infection; however, there is still much that we do not know. This study identifies host metabolic functions and proteins that play roles in HSV infection, using global quantitative stable isotope labeling by amino acids in cell culture (SILAC) proteomic profiling of the host cell combined with LC-MS/MS. We showed differential proteins during early, mid and late infection, using both cytosolic and nuclear fractions. We identified hundreds of differentially regulated proteins involved in fundamental cellular functions, including gene expression, DNA replication, inflammatory response, cell movement, cell death, and RNA post-transcriptional modification. Novel differentially regulated proteins in HSV infections include some previously identified in other virus systems, as well as fusion protein, involved in malignant liposarcoma (FUS) and hypoxia up-regulated 1 protein precursor (HYOU1), which have not been identified previously in any virus infection.

Caveolin-1 limits human influenza A virus (H1N1) propagation in mouse embryo-derived fibroblasts

Caveolin expression supports the multiplication of retro-, ortho- and paramyxoviruses in susceptible cells. However, human influenza A virus (IAV), an orthomyxovirus, does not multiply efficiently in mouse embryo fibroblasts (MEFs), which are abundant in caveolin-1 (Cav-1). Surprisingly, the absence of Cav-1 in a MEF cell line removed the block for IAV replication and raised the infectious titer 250-fold, whereas the re-introduction of Cav-1 reversed the effect. The monitoring of cellular pathways revealed that Cav-1 loss considerably increased activities of p53. Furthermore, infection of MEF Cav-1 (-/-) induced reactive oxygen species (ROS) and pronounced apoptosis in the late phase of viral multiplication, but no type I IFN response. Strikingly, pharmacological inactivation showed that the elevated levels of ROS together with apoptosis caused the increase of virus yield. Thus, Cav-1 represents a new negative regulator of IAV infection in MEF that diminishes IAV infectious titer by controlling virus-supportive pathways.